[laszlost]

I have been working on bringing a 1998 916 back to life after the previous owner started parting her out. This past weekend was the first time that I was able to take her on a couple of good rides and all I can think about is the next ride. However I am running into a problem after I roll off the throttle and then come back on with slight throttle it will sputter, this is usually under 4000 rpms and have noticed it coming off idle. It seems like if I come on with more throttle it doesnt seem to sputter. The interesting thing is that my buddy with a 05 Monster S4 has similar symptoms. Part of me wonders if it is just a characteristic of these motorcycles and is an issue that requires me to adjust my riding style. Ideally I would like to try and figure out the root of the problem and fix it. I know there has been a lot of discussion of installing a power commander and custom dyno tuning but I am wondering if I am missing something in my setup.

The history on the bike is that it came a Termignoni exhaust but without a computer so I replaced it with a unit from Gotham Cycles and a FIM chip. So I was really started from square one setting up the throttle position sensor and then moved to a throttle body sync. When it came time for the air bypass screw to set the idle to 1100 rpm, as described in the manual, it seemed like no matter what I did, no adjustment would effect the idle. In order to get the idle smooth I had to screw in the idle screw, what the manual says should be at ~310 mv reading from the throttle position sensor. Also when it came to setting the CO trim pot, I didn't have access to a exhaust gas analyzer so I had to roughly set it. It was initially running lean but after an adjustment it seemed to be running well and I am about right in the middle of the adjustment.

At this point I have the air bypass screw set to 1/4 turn from full closed and the idle set to 1200 rpm with the idle screw. It seems like it is almost running lean on really small throttle which from what I understand is controlled by the air bypass screw. Should I have the air bypass screw fully closed? How many turns do most people have on their setups? Is it ok to have the idle screw set the idle as it seems like the only way it will be stable? I have also played with small CO trim pot adjustments to see if it would effect it but the sputter was always there. I am open to any ideas or feedback.

Thanks,
David

 

[DukeDesmo]

Sounds like it's running lean to me. I set mine to 5% CO and whilst that may kill a few Polar Bears it runs nicely...

 

[sean197611]

Another 1998 916 in restoration, just like mine.

I had the similar issue during my 916 rebuild, the engine was doing exactly what you described at 4000rpm, especially under 1/2 throttle position. Sputter, back fire, loss of power.

I wouldn't worry too much with the air by-pass screws, I had mine set at 1/4 turn out from closed, idle speed at 1200rpm is just perfect.

Its difficult to set CO without gas analyzer, the easiest way to test is: Engine at idle speed, turn the idle mixture trimmer from Lean to Rich or vise versa, if the engine rpm starts to raise at particular direction or position, that would the direction of your fuel setup.

Good luck.

 

[laszlost]

sean19766,

Did you set the idle speed with the idle adjust screw or did you have that set at the recommended 316 mV? Or did the 1/4 turn or the air bypass screw put the idle speed at the right spot? I was able to pickup a gas analyzer on eBay but it was an older model so hopefully it will work out. I did set the mixture trimmer in a similar manner that you talked about, but do you know if yours is more on the lean or the rich side?

 

[DukeDesmo]

sean19766,

Did you set the idle speed with the idle adjust screw or did you have that set at the recommended 316 mV?

Set the idle with the idle screw, don't worry about the 300mv (or 316mv?) at idle this is completely irrelevant.

My bike is set to 150mv on closed throttle, 5% CO (both cylinders), throttles balanced and it idles beautifully at around 1100rpm at 190mv on the TPS, it pulls smooth as you like through the rev range, makes good power with no dips or sputters.

 

[yellow blur]

+1 on the lean condition and TPS settings as long as the TPS was set correctly with completely closed butterflies as the proper base setting, after thats set correctly use the idle stop screw to adjust idle speed.

One problem is any time you mess with the bypass screws, you also screw with the balance. This to can add to an ill running engine and the TB's should truly be balanced to "Fine" tune. Time to get the carb sticks out and get that right before doing any mixture settings. The bypass screws are only meant to lean out the richer running cylinder to match the leanest cyl. from the trimmer screw setting. The manual is confusing in that area.

 

[sean197611]

sean19766,

Did you set the idle speed with the idle adjust screw or did you have that set at the recommended 316 mV? Or did the 1/4 turn or the air bypass screw put the idle speed at the right spot? I was able to pickup a gas analyzer on eBay but it was an older model so hopefully it will work out. I did set the mixture trimmer in a similar manner that you talked about, but do you know if yours is more on the lean or the rich side?

Here are the procedures:

1. Set throttle bodies butterfly valves closed position, ensure its smooth motion from close to WOT.
2. Set TPS position to recommend 316mV or whatever other mV spec you think its best.
3. Connect vaccum gauges to inspect and sync throttle bodies, set fast idle using throttle lock, adjust sync by turning the knob on throttle linkage shaft.
4. Release fast idle, adjust idle air bypass screws, so idle smoothly.
5. Disconnect vaccum gauges, perform gas analyzer test, set idle mixture on ECU.
6. Set idle speed by adjusting idle stop screw.

I had Ultimap UM181 chip, so I end up on the rich side. By turning my idle mixture to full lean, I was still pumping 4.0%Co at idle. I end up using PCIII to build a fuel map for my bike, and now is running perfectly.